arXiv subjects

Shingo Kukita

Publications and source records attributed to Shingo Kukita.

18 recordsLinked to original sources

arXiv2026

Limits of independent and identical measurements for quantum illumination with an unknown return phase

Quantum illumination exploits entanglement between a transmitted signal and a retained idler to improve the error-probability exponent of target detection by roughly a factor of four (6 dB) over that with a coherent state of the same transmitted energy. This advantage presumes a known return phase. In practice, the phase is set by the range to the target and the condition of its surface, and is difficult to know in advance. Whether the advantage survives when this phase is unknown is not obvious. Here, we cast target detection as a composite hypothesis test in which the return phase is an unknown constant common to all trials, and we restrict the receiver to independent and identical measurements on each copy. We bound the worst-case error exponent at low reflectivity for every such measurement and every input state of a single signal mode and an idler of any dimension. We then show that unentangled coherent light with heterodyne detection already saturates this bound, for every value of the phase. Entanglement therefore confers no advantage in this setting. The class of independent and identical measurements contains many implementable quantum-illumination receivers, including the optical parametric amplifier and phase-conjugate receivers. Our result shows that none of them can offer a quantum advantage in the worst case over the phase, to leading order in the reflectivity.

quant-ph↗

arXiv2026

Suppressing Detuning-Induced Bias in Ramsey Magnetometry with Composite Pulses

Quantum sensing estimates a physical parameter encoded in the state of a probe; with independent spin probes the precision follows the standard quantum limit. Studies of sensing precision often assume that the parameters entering the model, such as the noise, are known. In practice these parameters are not always known, and a mismatch between the assumed and actual values induces a systematic error. Here we study single-qubit Ramsey magnetometry of a DC magnetic field under an unknown detuning between the actual and nominal spin frequencies: A first pulse puts the qubit into a superposition of its two states, the field to be sensed then adds a relative phase during an exposure stage, and a second pulse enables the readout. In our setting, the field acts only during the exposure stage, whereas the detuning acts throughout the whole protocol. We analyze how the detuning biases the estimate, preventing the total estimation error from following the standard quantum limit. We then construct a composite-pulse preparation and readout that exploits the difference in the intervals over which the field and the detuning act to cancel the detuning to first order. We evaluate the performance of this composite-pulse protocol and show that it suppresses the detuning-induced bias.

quant-ph↗

arXiv2026

Mitigating Detuning-Induced Systematic Errors in Entanglement-Enhanced Metrology

Quantum sensing leverages non-classical resources to enhance precision. In particular, Greenberger-Horne-Zeilinger (GHZ) states can, in principle, attain the Heisenberg limit that surpasses the standard quantum limit. While many studies have examined how open-system noise-typically modeled with Lindblad master equations-degrades GHZ-based metrology, coherent control imperfections during state preparation and readout have received less attention. Here, we analyze the effect of detuning between actual and nominal spin frequencies in a GHZ-state preparation scheme employing a frequency selective pulse. We show that detuning induces coherent, systematic error that prevents GHZ sensing from reaching the Heisenberg limit. To mitigate this effect, we design a composite-pulse protocol that compensates for detuning-induced errors and improves the sensitivity under the effect of coherent error.

quant-ph↗

arXiv2025

Geometric Construction of Dynamically Corrected Quantum Gates

The foundation of quantum technologies lies in the precise control of quantum systems. It is crucial to implement dynamically corrected quantum gates (DCQG), which compensate for individual quantum gate errors to make them more resilient to errors alongside quantum error correction. Off-resonance error (ORE), which originates from fluctuation and mis-calibration of resonance frequencies of qubits, is one of the most critical error types to be compensated. There have been many studies on constructing DCQGs robust against ORE up to its first order.Explicit construction of second-order robust DCQGs against ORE has been discussed less. Recently, the geometric meaning of the second-order robustness against ORE was uncovered. From this implication, we propose a geometric construction of second-order DCQGs against ORE using a first-order DCQG as a seed.

quant-ph↗

arXiv2024

Optimal quantum controls robust against detuning error

Precise control of quantum systems is one of the most important milestones for achieving practical quantum technologies, such as computation, sensing, and communication. Several factors deteriorate the control precision and thus their suppression is strongly demanded. One of the dominant factors is systematic errors, which are caused by discord between an expected parameter in control and its actual value. Error-robust control sequences, known as composite pulses, have been invented in the field of nuclear magnetic resonance (NMR). These sequences mainly focus on the suppression of errors in one-qubit control. The one-qubit control, which is the most fundamental in a wide range of quantum technologies, often suffers from detuning error. As there are many possible control sequences robust against the detuning error, it will practically be important to find ``optimal" robust controls with respect to several cost functions such as time required for operation, and pulse-area during the operation, which corresponds to the energy necessary for control. In this paper, we utilize the Pontryagin's maximum principle (PMP), a tool for solving optimization problems under inequality constraints, to solve the time and pulse-area optimization problems. We analytically obtain pulse-area optimal controls robust against the detuning error. Moreover, we found that short-CORPSE, which is the shortest known composite pulse so far, is a probable candidate of the time optimal solution according to the PMP. We evaluate the performance of the pulse-area optimal robust control and the short-CORPSE, comparing with that of the direct operation.

quant-ph↗

arXiv2024

Artificial Relaxation in NMR Experiment

Environmental noises cause the relaxation of quantum systems and decrease the precision of operations. Apprehending the relaxation mechanism via environmental noises is essential for building quantum technologies. Relaxations can be considered a process of information dissipation from the system into an environment with infinite degrees of freedom (DoF). According to this idea, a model of artificial relaxation has been proposed and demonstrated in NMR experiments. Although this model successfully understood the central idea of relaxation, we observed recursive behavior, which is non-ideal to describe relaxation, because of few DoF of the ``artificial environment''. In this paper, we extend the approach of the artificial environment and discuss, theoretically and experimentally, how many DoF of the environment are necessary for realizing ideal relaxation behavior. Our approach will help us thoroughly understand the concept of relaxation.

quant-ph↗

arXiv2022

Quantum Thermodynamics applied for Quantum Refrigerators cooling down a qubit

We discuss a quantum refrigerator to increase the ground state probability of a target qubit whose energy difference between the ground and excited states is less than the thermal energy of the environment. We consider two types of quantum refrigerators: (1) one extra qubit with frequent pulse operations and (2) two extra qubits without them. These two types of refrigerators are evaluated from the viewpoint of quantum thermodynamics. More specifically, we calculate the heat removed from the target qubit, the work done for the system, and the coefficient of performance (COP), the ratio between the heat ant the work. We show that the COP of the second type outperforms that of the first type. Our results are useful to design a high-performance quantum refrigerator cooling down a qubit.

quant-ph↗

arXiv2022

General off-resonance error robust symmetric composite pulses with three elementary operations

Accurate quantum control is a key technology for realizing quantum information processing, such as quantum communication and quantum computation. In reality, a quantum state under control suffers from undesirable effects caused by systematic errors. A composite pulse (CP) is used to eliminate the effects of systematic errors during control. One qubit control, which is the most fundamental in quantum control, is typically affected by two errors: pulse length error (PLE) and off-resonance error (ORE). In this study, we focus on ORE-robust CPs and systematically construct ORE-robust symmetric CPs with three elementary operations. We find an infinitely large number of ORE-robust CPs and evaluate their performance according to gate infidelity and operation time, both of which are important for the realization of accurate quantum control.

quant-ph↗

arXiv2021

Short composite rotation robust against two common systematic errors

Systematic errors hinder precise quantum control. Pulse length errors (PLEs) and off-resonance errors (OREs) are typical systematic errors that are encountered during one-qubit control. A composite pulse (CP) can help compensate for the effects of systematic errors during quantum operation. Several CPs that are robust against either PLE or ORE have been identified. However, few attempts have been made to construct CPs that are robust against both errors (bi-robust). We develop a novel bi-robust CP for one-qubit operations by modifying a PLE robust CP, which exhibits a shorter operation time than that of previously developed bi-robust CPs.

quant-ph↗

arXiv2021

Polarizing electron spins with a superconducting flux qubit

Electron spin resonance (ESR) is a useful tool to investigate properties of materials in magnetic fields where high spin polarization of target electron spins is required in order to obtain high sensitivity. However, the smaller magnetic fields becomes, the more difficult high polarization is passively obtained by thermalization. Here, we propose to employ a superconducting flux qubit (FQ) to polarize electron spins actively. We have to overcome a large energy difference between the FQ and electron spins for efficient energy transfer among them. For this purpose, we adopt a spin-lock technique on the FQ where the Rabi frequency associated with the spin-locking can match the resonance (Larmor) one of the electron spins. We find that adding dephasing on the spins is beneficial to obtain high polarization of them, because otherwise the electron spins are trapped in dark states that cannot be coupled with the FQ. We show that our scheme can achieve high polarization of electron spins in realistic experimental conditions.

quant-ph↗

arXiv2021

Heisenberg-limited quantum metrology using collective dephasing

The goal of quantum metrology is the precise estimation of parameters using quantum properties such as entanglement. This estimation usually consists of three steps: state preparation, time evolution during which information of the parameters is encoded in the state, and readout of the state. Decoherence during the time evolution typically degrades the performance of quantum metrology and is considered to be one of the major obstacles to realizing entanglement-enhanced sensing. We show, however, that under suitable conditions, this decoherence can be exploited to improve the sensitivity. Assume that we have two axes, and our aim is to estimate the relative angle between them. Our results reveal that the use of Markvoian collective dephasing to estimate the relative angle between the two directions affords Heisenberg-limited sensitivity. Moreover, our scheme based on Markvoian collective dephasing is robust against environmental noise, and it is possible to achieve the Heisenberg limit even under the effect of independent dephasing. Our counterintuitive results showing that the sensitivity is improved by using the decoherence pave the way to novel applications in quantum metrology.

quant-ph↗

arXiv2020

Controllable non-Markovianity in phase relaxation

Recently remarkable progress in quantum technology has been witnessed. In view of this it is important to investigate an open quantum system as a model of such quantum devices. Quantum devices often require extreme conditions such as very low temperature for the devices to operate. Dynamics can be non-Markovian in such a situation in contrast with Markovian dynamics in high temperature regime. This observation necessitates us to investigate a non-Markovian open quantum system, both theoretically and experimentally. In this paper, we report two important results: 1) Exact solution of a simple but non-trivial theoretical model and 2) demonstration of this model by NMR experiments, where non-Markovianity is continuously controllable. We observe qualitative agreement between theory and experiment.

quant-ph↗

arXiv2019

An upper bound on the number of compatible parameters in simultaneous quantum estimation

Simultaneous estimation of multiple parameters is required in many practical applications. A lower bound on the variance of simultaneous estimation is given by the quantum Fisher information matrix. This lower bound is, however, not necessarily achievable. There exists a necessary and sufficient condition for its achievability. It is unknown how many parameters can be estimated while satisfying this condition. In this paper, we analyse an upper bound on the number of such parameters through linear-algebraic techniques. This upper bound depends on the algebraic structure of the quantum system used as a probe. We explicitly calculate this bound for two quantum systems: single qubit and two-qubit X-states.

quant-ph↗

arXiv2018

Parameter estimation by decoherence in the double-slit experiment

We discuss a parameter estimation problem using quantum decoherece in the double-slit interferometer. We consider a particle coupled to a massive scalar field after the particle passing through the double slit and solve the dynamics non-perturbatively for the coupling by the WKB approximation. This allows us to analyze the estimation problem which cannot be treated by master equation used in the research of quantum probe. In this model, the scalar field reduces the interference fringes of the particle and the fringe pattern depends on the field mass and coupling. To evaluate the contrast and the estimation precision obtained from the pattern, we introduce the interferometric visibility and the Fisher information matrix of the field mass and coupling. For the fringe pattern observed on the distant screen, we derive a simple relation between the visibility and the Fisher matrix. Also, focusing on the estimation precision of the mass, we find that the Fisher information characterizes the wave-particle duality in the double-slit interferometer.

quant-ph↗

arXiv2017

Entanglement dynamics in de Sitter spacetime

We apply the master equation with dynamical coarse graining approximation to a pair of detectors interacting with a scalar field. By solving the master equation numerically, we investigate evolution of negativity between comoving detectors in de Sitter space. For the massless conformal scalar field, it is found that a pair of detectors can perceive entanglement beyond the Hubble horizon scale if the initial separation of detectors is sufficiently small. At the same time, violation of the Bell-CHSH inequality on the super horizon scale is also detected. For the massless minimal scalar field, on the other hand, the entanglement decays within Hubble time scale owing to the quantum noise caused by particle creations in de Sitter space and the entanglement on the super horizon scale cannot be detected.

gr-qc↗

arXiv2017

Harvesting large scale entanglement in de Sitter space with multiple detectors

We consider entanglement harvesting in de Sitter space using a model of multiple qubit detectors. We obtain the formula of the entanglement negativity for this system. Applying the obtained formula, we find that it is possible to access to the entanglement on the super horizon scale if sufficiently large number of detectors are prepared. This result indicates the effect of the multipartite entanglement is crucial for detection of large scale entanglement in de Sitter space.

gr-qc↗

arXiv2017

Perturbative Dynamics of Open Quantum Systems by Renormalization Group Method

We analyze perturbative dynamics of a composite system consisting of a quantum mechanical system and an environment by the renormalization group (RG) method. The solution obtained from the RG method has no secular terms and approximates the exact solution for a long time interval. Moreover, the RG method causes a reduction of the dynamics of the composite system under some assumptions. We show that this reduced dynamics is closely related to a quantum master equation for the quantum mechanical system. Then, we compare this dynamics with the exact dynamics in an exactly solvable spin-boson model.

quant-ph↗

arXiv2016

Derivation of Markovian master equation by renormalization group method

We present a derivation of the Markovian master equation by the renormalization group method. Starting from a naive perturbative solution of the von Neumann equation, the reduced density matrix with the coarse grained time steps is obtained using the assumption of short correlation time of the bath field. Then by applying the renormalization group method, we show that the dependence of the specific initial time on the perturbative solution can be removed and the Markovian semigroup master equation in the Gorini--Kossakowski--Lindblad--Sudarshan (GKLS) form is obtained in the weak coupling limit.

quant-ph↗